If I down shift my car all the time is it bad for the clutch or tran? I have dct.

if you're doing it when not appropriate then yea i think that would be unwanted strain on the transmission. Shift at the appropriate times and itll be fine. downshifting is a natural process for a transmission such as ours

Actually if done properly it is actually beneficial. Tranny gears normally wear on just one side under load, when down shifting, from my understanding it keeps an even wear on gears. Also the higher revs help in keeping the combustion chamber clean as during the engine deceleration the car is lean. Keep in mind if you don't rev match properly you put undo strain on the synchros as well as the clutch. Since you have a DCT the computer will make sure that the downshift will be performed correctly.

^^^ FWIW, wearing both sides of the gear teeth causes more backlash in the gears which is undesirable. Engine braking really has no effect on keeping the combustion chambers clean. Most modern OEM EFI designs shut off the fuel on closed throttle deceleration (overrun), so only air is run thru the engine until the throttle is opened or the engine is close to idle speed.

That being said down shifting at the proper RPM is just fine and normal. The slight increase in wear of the clutch and trans is normal. Abusive down shifting is totally different and to be avoided.